This invention relates to apparatus for the preparation of semiconductor material in general and more particularly to a temperature measuring arrangement for use in measuring the temperature of a granular semiconductor material which is being evaporated within an ampoule for the formation of semiconductor material.
It is well known that dense polycrystalline gallium phosphide can be produced from the two components at a reaction temperature of 1450.degree.C by heating gallium in a closed tube and passing vaporous phosphorus through holes in the tube wall to the area of the heated gallium whereupon it will react to form gallium phosphide. In such a method the gallium is advanced at a comparatively low travel speed of approximately 1 cm/hr through a heating zone to result in gallium phosphide which contains a carbon content of approximately 1000 ppm. In such an arrangement the operating pressure of the installation, which is essentially determined by the vapor pressure of the phosphorus, amounts to approximately 8-10 atm. [See J. Cryst, Growth 3,4 286 (1968)]. Another known method for the direct synthesis of polycrystalline gallium phosphide using gallium and phosphorus at a temperature of about 1500.degree.C and a pressure of 6-35 bar is disclosed by Frosch and Derick in Journal of the Electrochemical Society, Vol. 108, p. 251, 1961. The components of the semiconductor compound are placed in a quartz tube or ampoule which is arranged in a furnace and is provided with a high frequency heating device. The high frequency heating device is inductively coupled to a graphite boat located in the tube and containing one of the components, e.g. gallium. The phosphorus is placed at the end of the ampoule with the space containing the phosphorus closed off with quartz wool or the like to permit the phosphorus vapor to reach the gallium. Typically, this phosphorus vapor is produced at a temperature of about 480.degree.-580.degree.C. The pressure within the ampoule in this horizontal system depends on the temperature and can vary between 6 and 35 atm. The quartz ampoule is installed within an autoclave which has an elevated pressure therein which is ideally at the operating pressure of the phosphorus in the ampoule. By thus applying an external pressure to the walls of the ampoule the internal pressure of the phosphorus is counteracted and only a small differential pressure results. As a result the quartz ampoule need not be designed for a large pressure differential. For reacting, the boat with the semiconductor component such as gallium is moved through the inductively heated zone of elevated temperature within the heating device. The portion of the ampoule containing the phosphorus is typically installed within an oven which raises it to the evaporation temperature. Through this apparatus a dense gallium phosphide which will contain a small piece of gallium after the process is ended is formed.
In the direct synthesis of a semiconductor compound, made up of one component having a substantially higher partial pressure than the other or others, phosphorus and arsenic are suitable, for example, as the components with the high partial vapor pressure. For reaction therewith the components, gallium and indium are suitable. For the production of gallium phosphide, operation will typically be in a temperature range of 1000.degree.-1400.degree.C and preferably at 9-10 atm. The boat containing the gallium is generally made of graphite and may also be made of boron nitride or quartz.
The quartz ampoule is generally used for a single synthesis. A direct measure of the operating pressure within the quartz ampoule using a pressure measuring device and the necessary associated pressure-tight sealed feedthroughs becomes comparatively expensive. Because the vapor pressure of the phosphorus varies directly as a function of temperature in a well known manner, the operating temperature within the ampoule will essentially correspond to the vapor pressure of the phosphorus. The phosphorus vapor temperature can be determined indirectly by determining the temperature of the granular phosphorus. There is a problem, however, in that the temperature of the phosphorus is determined not only by the temperature of the heating device, i.e. the oven surrounding it, but is also a function of the radiation heat generated from the boat, and which is raised to a temperature within the reaction range of the gallium phosphide synthesis and is, thus, much higher than the phosphorus temperature. Furthermore, the wall of the ampoule also radiates heat. This radiated heat is further increased by the fact that the end of the ampoule typically rests on a spoon-shaped support normally consisting of a metal such as stainless steel. As a result the determination of the phosphorus temperature through measurement of the ampoule wall is not possible.
In view of these difficulties the need for an arrangement which permits accurately determining the temperature of the evaporating semiconductor material and which can then be used to regulate the pressure and which measurement means can be installed independent of the ampoule used in the synthesis becomes evident.